1. Field of the Invention
The present invention generally relates to methods for etching a magnetic memory cell stack, and more particularly to methods for etching a magnetic tunnel junction (MTJ) magnetic random access memory (MRAM) cell stack.
2. Description of the Related Art
The push to continually increase memory bit densities has lead to memory structures to replace conventional memory cells having sub-micron feature size. Advances in giant magnetoresistance (GMR) and colossal magnetoresistance (CMR) materials have lead to potential substitutes for conventional memory cells, including, but not limited to, substitutes for static random access memory (SRAM) cells, dynamic random access memory (DRAM) cells, and flash memory cells, among others. Moreover, owing to a non-volatile aspect of magnetic memory, it is a potential substitute for read only memory (ROM) technologies too.
GMR material is formed as material layers to provide a memory cell stack. However, GMR memories tend to have resistance limitations creating sensing inefficiency. To address these limitations, a magnetic tunnel junction (MTJ) stack has been proposed. In an MTJ stack, a tunnel barrier layer separates two magnetically oriented material layers.
Forming such magnetic memory cell stacks is problematic owing to non-volatility of materials used, therefore, ion milling has been used to form magnetic memory cell stacks. However, ion milling is not material selective, making stopping at a correct depth problematic, and etch rates for ion milling processes are too slow to be commercially practical. Moreover, temperatures associated with ion bombardment negatively affects magnetic properties of magnetic materials. Accordingly, plasma or dry etching using predominantly chemically active etchants is desirable for etching magnetic materials owing to etch rate and selectivity, as well as lower temperature as compared with ion milling.
Etchants used to etch such magnetic memory cell stacks are quite aggressive, and thus etching by-products of such etchants tend to be corrosive. Moreover, Pt, Mn, Ni, Fe, and Co among other materials used in magnetic memory cell stacks tend to etch slower than photoresist is consumed. In other words, resist thickness is increased to etch through a magnetic memory cell stack, which in turn increases aspect ratio. An increase in aspect ratio can preclude dense spacing of magnetic memory cell stacks due to difficulty in evacuating by-products. Furthermore, etching of such stacks in densely spaced areas may be prematurely halted if aspect ratio is too high.
In an article entitled xe2x80x9cRelative merits of Cl2 and CO/NH3 plasma chemistries for dry etching magnetic random access memory device elementsxe2x80x9d by K. B. Jung et al. in Volume 85, Number 8, of the Apr. 15, 1999, issue of the Journal of Applied Physics at pages 4788 to 4790, high ion density plasma reactors, such as electron cyclotron resonance (ECR) and inductively coupled plasma (ICP) reactors, were discussed for etching magnetic multilayers. More particularly, an ICP source was used to etch magnetic multilayers using a Cl2 with Ar, N2, H2 and CO/NH3 chemistries. A mentioned drawback of this chemistry was corrosion of magnetic materials from chlorine residue, though it is alleged that a small but measurable enhancement resulted from the CO/NH3 plasma. However, with CO/NH3, etch rates of photoresist were a factor of ten higher, requiring use of a hard mask, such as an SiO2 mask. In an article entitled xe2x80x9cDevelopment of chemically assisted dry etching methods for magnetic device structuresxe2x80x9d by K. B. Jung et al. in Volume 17, Number 6, of the November/December 1999, issue of the Journal of Vacuum Science Technology at pages 3186 to 3189, post-etch rinsing in water or in situ cleaning with H2, O2 or SF6 plasma discharges were suggested as solutions for removing post-etch chlorine residues from a Cl2/Ar etch chemistry. Where exposure to the O2 plasma was alleged to be the only one to harm magnetic properties of MRAM stacks of the three suggested in situ cleaning plasmas.
Accordingly, it would be desirable and useful to provide method and apparatus to etch magnetic material stacks that at least reduces corrosion. Moreover, it would be more desirable and useful if such method and apparatus were more selective to mask resist.
An aspect of the present invention is a method for forming a magnetic memory cell. A semiconductor wafer is loaded into an etch process chamber. The semiconductor wafer has at least one masking layer formed over a set of layers for forming the magnetic memory cell. The set of layers comprises a subset of layers including at least two magnetic layers separated by an electron barrier layer and including an anti-magnetic layer. Plasma etch is done through openings formed in the at least one masking layer. The plasma etching includes flowing at least one plasma source gas into the etch process chamber, where the at least one plasma source gas comprises HCl.
Another aspect of the present invention is a method for plasma etching in a plasma reactor to provide a magnetic memory cell stack. A work piece is positioned in the plasma reactor. The work piece comprises at least one magnetic material layer that is selected from NiFe, CoFe, NiFeCo, and Ru. A plasma source material is flowed into the plasma reactor chamber. The plasma source material comprises HCl from which a plasma from the plasma source material is generated. The work piece is exposed to the plasma to etch the at least one magnetic material layer.
Another aspect of the present invention is a method for plasma etching in a plasma reactor to provide a magnetic memory cell stack. A work piece is positioned in the plasma reactor. The work piece comprises an anti-magnetic material layer which is selected from IrMn and PtMn. A plasma source material is flowed into the plasma reactor chamber. The plasma source material comprises HCl from which a plasma from the plasma source material is generated. The work piece is exposed to the plasma to etch the anti-magnetic material layer.
Another aspect of the present invention is a method for forming a magnetic memory cell in two etch process chambers. More particularly, a semiconductor wafer is loaded into a first etch process chamber. The semiconductor wafer has at least one masking layer formed over a set of layers for forming the magnetic memory cell. The set of layers includes a subset of layers having at least two magnetic layers separated by an electron barrier layer and include an anti-magnetic layer. The subset of layers does not include a diffusion barrier layer, though a diffusion barrier layer can be located below the subset of layers. The subset of layers is plasma etched through openings formed in the at least one masking layer, where the plasma etching includes flowing a first plasma source gas comprising HCl into the first etch process chamber. The semiconductor wafer is then removed from the first etch process chamber and loaded into a second etch process chamber for plasma etching the diffusion barrier layer of the semiconductor wafer, where a second plasma source gas comprising CHF3 is flowed into the second etch process chamber.
Another aspect of the present invention is for a process chamber configured to allow an operator thereof to select a gaseous mixture for etching a portion of a magnetic memory cell stack. The portion of the magnetic memory cell stack has two magnetic orientation material layers separated by a tunnel barrier layer and has an anti-magnetic material layer. The gaseous mixture comprises HCl as a main etchant gas to etch the portion of the magnetic memory cell stack.